Neurodegenerative diseases have a major impact on society. For example, approximately 3 to 4 million Americans are afflicted with a chronic neurodegenerative disease known as Alzheimer's disease. Other examples of chronic neurodegenerative diseases include diabetic peripheral neuropathy, multiple sclerosis, amyotrophic lateral sclerosis, Huntingdon's disease and Parkinson's disease. Not all neurodegenerative diseases are chronic. Some acute neurodegenerative diseases include stroke, schizophrenia, and epilepsy as well as hypoglycemia and trauma resulting in injury of the brain, peripheral nerves or spinal cord. There is a need for improved therapeutic agents and methods for reversing or retarding neuronal damage associated with each of these conditions.
Neurodegenerative diseases and aging are characterized by a wide range of symptoms which vary in severity and range from individual to individual. For example, Alzheimer's disease is characterized by symptoms such as depression, aggression, impairment in short-term memory, impairment in intellectual ability, agitation, irritability and restlessness. Since estrogen becomes deficient in post-menopausal women, and since estrogen is believed to affect mood, some studies have been undertaken to assess the relief of behavioral symptoms associated with Alzheimer's disease. Unfortunately, those clinical trials that have been performed to establish the beneficial effect of estrogen on Alzheimer's disease have concluded that no statistically significant improvements in the disease course or symptoms resulted from the treatment. Fillet et al. 1986, Psychoneuroendocrinology 11:337-345; Honjo et al. 1989, Steroid Biochemistry 34:521-524. In one study where only 1 female and 1 male patient were studied and no statistics were available, a rapid reduction in symptoms of senile dementia was observed when estrogen was administered to the female patient in a cocktail of drugs together with chorionic gonadotrophin, a vasodilator and a non-steroidal anti-inflammatory agent after a period as short as one week (Aroonsakul 1990, U.S. Pat. No. 4,897,389). There is a need for a better understanding of the underlying process of neurodegeneration such that improved treatment protocols and effective drugs may be designed that are effective at treating the disease itself so as to bring about a longterm meaningful reversal of symptoms.
A common feature of neurodegenerative disorders and the process of aging in animals is the progressive cell damage of neurons within the central nervous system (CNS) leading to loss of neuronal activity and cell death. This loss of activity has been correlated with adverse behavioral symptoms including memory loss and cognitive deficits. Therapeutic agents that have been developed to retard loss of neuronal activity either have toxic side effects or are prevented from reaching their target site because of their inability to cross the blood-brain barrier. The blood-brain barrier is a complex of morphological and enzymatic components that retards the passage of both large and charged small molecules thereby limiting access to cells of the brain. There is a need for novel therapeutic agents that are readily transported across the blood-brain barrier as well as for novel methods of treatment of neurodegenerative disorders that directly target the damaged site and are non-toxic.
Traditional methods of treating neurological symptoms focus on; modifying the electrical impulse itself as it moves between and along neurons; or modifying the release or degradation of neurotransmitters. It is now recognized that neuronal cell density has an important impact on function. In various pathological conditions, loss of cell density has been observed resulting from accelerated neuronal cell death. The pattern of degeneration of neurons typically originates from the nerve terminals and progresses "backward" toward the cell body (retrograde degeneration). In several systems, lesioning of certain brain regions results in compensatory sprouting of axons. This plasticity of neurons is attributed at least in part to the presence of trophic growth factors.
These findings have spurred efforts to identify therapeutic agents that compensate for cell loss by stimulating sprouting of dendrites and axons of remaining cells so as to improve the structural integrity of the damaged region. However, the optimal density of neurons and neuronal extensions is a delicate balance between deficiency and excess, a balance that varies with the environment of the cells. This balance can be disrupted when therapeutic agents act on normal or inappropriate tissue. There is a need therefore to target therapeutic agents at a therapeutic dose specifically to those regions where they are required, or, alternatively, to identify agents that have a natural specificity for the target site only.
To date, there are no safe and effective methods for treating loss of neuronal activity. However, considerable attention has recently been focused on naturally occurring proteins, collectively called neurotrophic factors (see Table I), that promote growth and maintenance of cells of the central nervous system (CNS) and sympathetic and sensory neurons of the peripheral nervous system. In particular, the administration of nerve growth factor (NGF), a protein which is normally transported retrogradely in the intact brain from the hippocampus to the septal cholinergic cell bodies as well as from the cortex to the nucleus basalis, provides trophic support to cholinergic neurons and has been shown in animal models to have utility in reducing the effects of neurodegeneration due to trauma, disease or aging. The septum and the nucleus basalis are part of a region of the brain known as the basal forebrain. The effectiveness of administering NGF in response to damage is supported by experiments that demonstrate that cholinergic neurons in the medial septum can be protected from retrograde degeneration by chronic infusion of exogenous NGF (Rosenberg et al. 1988, Science 242:1575-1578). Indeed, infusion of NGF has been shown to significantly attenuate retrograde degeneration of cholinergic neurons after transection of their connections in the fimbria (the septo-hippocampal pathway).
One of the major problems confronting the use of NGF as a therapeutic agent is finding an appropriate method of increasing the levels of NGF at the appropriate target site. NGF is a large molecule and as such cannot normally pass across the blood-brain barrier and therefore has very limited access to the cells of the brain. Invasive methods are commonly used to place externally administered NGF within the brain. These methods are not sufficient to target NGF specifically to those cells where it is required. Non-localized targeting not only decreases the amount of protein available at the target site but also results in stimulation of growth of neurons at inappropriate sites resulting in potential harmful effects for the subject.
Another disadvantage of administering NGF as a therapeutic agent is that of induction of an immunological response to this protein. There is a need therefore for a compound that does not in itself cause an immune response but could stimulate the production of endogenous NGF.
Current methods for administering nerve growth factor across the blood-brain barrier include: polymeric implants, osmotic minipumps, cell therapy using genetically engineered autologous or heterologous cells secreting NGF for implantation into the brain, and methods of increasing the permeability of the blood-brain barrier thereby allowing diffusion of these molecules to cells in the brain. Where exogenous NGF is used, a relatively large amount of relatively costly recombinant protein is required.
Rather than these aforestated solutions to delivery of proteins, it would be desirable to: avoid invasive techniques; to control the amount and the site of delivery of neurotrophic proteins to sites where they are most needed thereby minimizing toxic side effects; and to minimize the health care costs of treatment.
An additional approach to treating neurological symptoms has followed the observation that certain amino acids (glutamic acid and aspartic acid) act as excitatory neurotransmitters that bind the N-methyl D-aspartate (NMDA) receptor. Excess release of these amino acids (EAA) causes overstimulation of the neurons in neurodegenerative diseases as well as in conditions of hypoglycemia or trauma, resulting in neuronal loss and behavioral dysfunctions. NMDA is a potent and toxic analogue of glutamate which has been shown in animal studies to mediate much of the neuronal death associated with head trauma, hypoglycemia, anoxia, hypoxia and other conditions, and compromises the flow of blood, oxygen or glucose to the central nervous system.
A number of synthetic compounds that act as antagonists of the receptor have been described and tested in animal models. The possibility that these compounds are toxic in humans remains unresolved. Despite many years of clinical research, these antagonists are not as yet available as therapeutic products for treating patients.
For the foregoing reasons there is a need for methods of protecting neurons from accelerated cell death caused by trauma or disease or by the aging process or by combinations of these factors. There is also a need for methods that stimulate the production of neurotrophic growth factors using small molecules that are capable of crossing the blood-brain barrier and that have minimal side effects.